FIG. 1 illustrates a conventional refrigeration system 10 (refrigeration cycle) for both sub-critical and transcritical refrigeration cycles. The refrigeration system 10 includes a throttle valve 14, an evaporator 16, a compressor 18, and a condenser (for sub-critical cycle) or gas cooler (for transcritical cycle) 22, all of which are in fluid communication with one another via a manifold 12. The refrigeration system 10 includes a working fluid that flows through the system and is used to remove thermal energy from the evaporator 16. FIG. 2 illustrates a thermodynamic diagram (cycle) for the conventional refrigeration system shown in FIG. 1, where the cycle positions/states (e.g., “a”, “b”, “c”, and “d”) corresponds to the schematic in FIG. 1. The cycle is a transcritical cycle because all states of the cycle are in the vicinity of the critical point of the working substance (e.g., CO2) with the throttling process proceeding from the supercritical pressure (Pa>Pcritical) to sub-critical pressure (Pb<Pcritical) at constant enthalpy in the vicinity of the critical enthalpy (ha=hb=hthrottle˜hcritical).
In an ideal (reversible) case, the conventional transcritical refrigeration system operates in the following way. From position “a” to position “b” is an isoenthalpic (constant enthalpy h=constant˜hcritical) throttling process from the supercritical fluid (Pa>Pcritical) state “a” to the sub-critical (Pb<Pcritical) liquid/vapor mixture state “b”.
From position “b” to position “c” is an isobaric (constant pressure Pb=Pc=constant<Pcritical) evaporation (phase change) process from the liquid/vapor mixture state “b” to the saturated (or possibly slightly superheated) vapor state “c”. During this process, heat is being absorbed by the working fluid in an evaporator to enable refrigeration.
From position “c” to position “d” is a compression process (in an idealized reversible case, isoentropically) from the saturated (or possibly slightly superheated) vapor state “c” at lower pressure Pc to the higher pressure Pd superheated vapor state “d”, which is also in the supercritical fluid domain.
From position “d” to position “a” is an isobaric (constant pressure Pd=Pa=constant>Pcritical) cooling of the working substance from the supercritical fluid state “d” with higher enthalpy (hd) to another supercritical fluid state “a” with lower enthalpy (ha). During this process, heat is being rejected to the atmosphere in the gas cooler.
Early in the 20th century, carbon dioxide was introduced and became popular as a refrigerant fluid (working fluid) because of its low toxicity, non-flammability, low cost, and universal availability. The use of competing refrigerants such as ammonia, sulfur dioxide, methylene chloride, and others, achieved much higher cycle efficiencies (i.e., coefficient of performance (COP)), but the applications were limited because of various other shortcomings. The use of CO2 as a refrigerant declined dramatically in the early 1930s, with development of chlorofluorocarbons (CFC) featuring low toxicity, as well as high COP of the refrigeration cycle.
Recently, the interest in carbon dioxide based refrigeration has picked up again, and quite sharply, owing to the ban on the use of CFCs and the phaseout of hydro-CFC (HCFC) due to serious environmental problems. Despite its unique advantages (e.g., low toxicity, non-flammability, low cost, environmental friendliness, and universal availability), low cycle efficiency is the major factor that prevents widespread application of CO2 refrigeration technology. This is an equally valid point for both a conventional vapor-compression cycle, as well as more recent supercritical/transcritical refrigeration cycles (critical temperature Tcritical=31.1° C. for carbon dioxide). For example, according to an ASHRAE Handbook (p. 167, 1993), the CO2 refrigeration cycle with an evaporating temperature of −15° C. and a condensing temperature of 30° C. has coefficient of performance (COP) of only 2.81, as compared to 4.77 for ammonia, 4.67 for R-22, and 4.41 for R-134a.
Therefore, there is a need in the industry to develop technology to overcome at least some of the deficiencies and inadequacies described above.